Was there new drivers or software installed recently? Try booting through safemode to see if this happens again. If so, then there is a problem in the registry. From this point you would have to reinstall the OS.
These are the first 2 things I would look at: Are the computers plugged in to a steady power supply like an UPS? Spiking and brownouts for only fractions of a second can cause reboots and destroy components. Are the power supplies on these machines too undersized for what you have installed on them? Some older machines come with only a 250 watt power supply so by the time you hook up, say 1 or 2 harddrives, a CD, a CD burner, good graphics and sound cards,USB devices or whatever, they are overloaded. 350 to 400 watt power supplies are fairly cheap and will give you enough power to run several add-ons without causing reboots. Hope this helps.
A big probability is that you have been infected by mblast virus. Kindly check the ff:
1. run msconfig and check whether the word “blast” is in your startup program, if it is so, remove it by downloading the blast removal tool from other PC.
2. update your virus info file.
It could be that the capacitors on the motherboard. I have had 3 IBM unit this year that had to be replaced at work.
Look at the capacitors, if they are flat on top, they are good. If they are raised they are bad.